1. Field of the Invention
This invention relates to a marking kit which comprises material which is capable of forming a matrix of marking material together with glass beads for incorporation into such a matrix, to surface treated glass beads suitable for incorporation into such a matrix, and to a process for the surface treatment of such glass beads. The invention also extends to a reflective surface marking comprising a synthetic matrix incorporating glass beads, and to a process for the reflective marking of a surface.
2. Description of the Related Art
The invention is particularly, but not exclusively, concerned with the formation of reflective markings on pavements of various kinds. The most extensive use of such markings is probably as lane markings on roads and/or as indications of rights of way at intersections. Other examples of such markings are lines or other markings on paved aircraft runways and taxiways and lines indicating spaces in car parks. Another example is that of reflective paints which find wide use in road traffic signs.
Such markings are given their reflective properties by incorporating glass beads into a matrix. The glass beads may be solid or hollow glass spheres, having a mean diameter in the range from some tens of micrometers up to several millimeters. It will be appreciated that for reasons of mechanical resistance, pavement markings usually incorporate solid glass beads. When glass beads are added to a liquid material which is capable of forming the matrix, that matrix contains beads distributed within its mass and/or in surface layers. The liquid material to be used to form the matrix can be a molten material, or a solution or a dispersion and it can be polymerized or polymerizable.
The reason for using reflective markings is so that the markings may be clearly visible at night. Their high visibility is promoted by incorporating glass beads into the markings to reflect light given out by a vehicle's headlights back towards the driver of the vehicle. It is quite clear that in order to obtain this result, some glass beads at least must be exposed at the surface of the markings. However, if the beads are too exposed at the surface and do not have good adhesion to the marking, there is, in the case of pavement markings, a risk that they may be easily abraded from the marking by traffic. Eventually most of the beads will become removed so that the marking becomes poorly reflective and thus less visible at night. It is thus desirable that the beads are well embedded into the matrix of marking material and have strong adhesion to it. It can also be desirable that the beads are well distributed through the thickness of the matrix of marking material in order to maintain good reflectivity as long as possible. Indeed, as the matrix-forming material wears, layers of matrix are abraded releasing some beads. If beads are distributed through the thickness of the matrix, they will be exposed by this process and will maintain the reflectivity. On the other hand, if the reflective marking is to be used under circumstances in which it will not be exposed to such a high degree of wear, for example on road traffic signs, it is desirable that the beads should be mainly at the surface of the matrix of marking material, while being adequately bound to it to resist weathering, and that they should be embedded in the layer of matrix-forming material to the degree appropriate for optimal retro-reflection.
Some synthetic polymer matrices, especially for use in surface markings, which have recently appeared on the market, are relatively viscous in the unhardened state, for example because they contain a significant loading of solids. One such instance is materials for road markings in the form of small patches. When glass beads which are intended to be light reflective, are sprinkled on the unhardened matrix-forming material, they float on that material without penetrating sufficiently into the body of the material. The result is that the beads are easily removed from the matrix even by weak abrasion. If they are at the surface of the matrix to make it reflective, the retro-reflection obtained can be inadequate or can quickly become so.